The present invention relates to an image-retransferable sheet for a dry image-transferring material capable of transferring an image such as letters, signs, figures or the like onto the surface of a desired substance by applying pressure to the image-transferring material whose image-bearing surface is brought into contact with the surface of the substance (hereafter referred to as "pressure-sensitive retransfer"). More specifically, it relates to an image-retransferable sheet of such a dry image-transferring material produced by thermally printing or transferring an image on the sheet with a thermally transferring type printing device such as a printer, a typewriter, a word processor or the like (hereafter referred to as "heat-sensitive transfer").
As base sheet (image-retransferable sheets) for dry image-transferring materials produced by heat-sensitive transfer, Japanese Patent Application No. sho-61-275537 discloses films of polyethylene, polypropylene, fluorine-containing resins, etc., or silicone resin-coated sheets of paper, metal foils, plastic films, etc., which have a smooth surface and exhibit a water-contact angle of at least 95 degree and preferably at least 105 degree.
In formation of an ink image on such a base sheet having a water-contact angle of 95 degree or more by heat-sensitive transfer, an ink temperature is necessarily increased to reduce surface tension of the ink and wet the surface of the base sheet to an extent that adhesion of the ink to the base sheet becomes larger than cohesive force of the ink and adhesion of the ink to an ink-donating base film such as PET. Thus, a high thermal energy is required for the image formation, and it is very disadvantageous to heat-sensitive transferring devices concerning durability of a thermal head and load to an electric source.
Further, ink images formed on such a surface of poor wettability are easily retansferred merely with little pressure applied thereto because of its poor adhesion to the surface so that, upon retransfer of certain portions of the images, the other portions of the images are undesirably retransferred and stain the intended images. Mere touch often removes images from the surface of poor wettability.
Furthermore, when the base sheet having a thermally transferred ink image is subjected to pressure-sensitive retransfer, the sheet is not easily fixed on an image-receiving substance since it has an extremely small coefficient of static friction, resulting in retransfer of imperfect images getting out of position or with distortion.
In order to prevent the base sheet from slipping, it is described in Japanese Patent Application No. sho-62-80127 to provide a sticky layer apart from thermally transferred images on the base sheet. However, an additional means required to provide such a sticky layer at predetermined portions of the base sheet, and a device for the above purpose is needed. Further, the sticky layer has to be covered with a separable sheet, etc. before use, requiring further additional means and costs. In the case of using a silicone resin-coated sheets as a base sheet as described above, two layers, i.e., the silicone resin layer and the sticky layer must be provided and it may well be that one of the two layers previously coated has influence on the other. That is, when a silicone resin is first coated on a sheet, a sticky material is repelled when coated on the silicone resin layer. When the sticky material is first coated at portions of a sheet, its stickiness makes it difficult to coat the silicone resin on the sheet. Even if a separable sheet is provided on the sticky layer, difficulty in coating of the silicone resin cannot be eased because of the increased thickness at the sticky layer-formed portions. Even with the two layers properly coated, fixation of the sheet is yet insufficient as the sticky layer exists only in portions not fully surrounding areas to which thermally transferred images are provided.
In any case, a surface treating agent like a silicone resin coated on a conventional base sheet is not transferred with an ink image but left as it is, and it functions to ensure improved releasability of the ink image from the sheet. Therefore, the surface treating agent has hitherto been selected or formulated to have the property of reducing wettability of the sheet and decreasing adhesion of the ink image to the sheet, which property, however, deteriorates the image-receiving property and image-rubbing resistance of the sheet and necessitate a high thermal energy.
In heat-sensitive transfer, a white paper or the like are generally used as a sheet to which an ink is thermally transferred. In the case, a large hiding power of images (such as letters, figures, etc.) is not required, and white-color images are not generally formed on the image-receiving substances. On the other hand, a large hiding power is sometimes needed in the aforesaid dry image-transferring system since various kinds of materials having various colors are contemplated as an image-receiving substance. In general, color images (e.g., white-color images) other than black images do not possess a large hiding power and often meet with difficulty in confirming the completion of retransfer on the image-receiving substances.